This Skwala nymph still has a couple months left to go before hatching, but it's still a good representative of its species, which was extremely abundant in my sample for a stonefly of this size. It's obvious why the Yakima is known for its Skwala hatch.
These are pretty much always called Worms and Leeches.
Worms and leeches are surprisingly common in trout streams, drifting on their own and not thrown from somebody's bait bucket. They often turn up in trout stomach samples.